Gamma correction provides for an accurately depicted display image on a display device. More specifically, Gamma correction controls the overall brightness of an image. Images which are not properly corrected may appear bleached out, or too dark. Gamma correction affects not only the brightness, however, but also the ratios of red to green to blue. Gamma correction is applied to a voltage response curve used to render images in various instances to compensate for differences in intensity. That is, a range of voltages sent to a display can generally be less than the desired or required voltages for correct display of an image. As such, the voltages require correction to more accurately depict the image. Since the relationship between the voltage sent to the display and the intensity which the display produces are known, a signal can be corrected by gamma correction before it gets to the display.
In conventional systems, input source images have associated with them assumed gamma characteristics for a display (e.g., Rec. 709 gamma, which is a transfer function known in the art). However, most displays depict different gamma characteristics from those assumed, such that input images are not correctly depicted on the display. A conventional method of characterizing a display includes measuring patches on the display using, for example, a spectroradiometer. In particular, for gamma characteristics, a series of patches are measured (this is called a ramp—e.g., a gray ramp is (Red, Green, Blue)=(0,0,0), (32,32,32), (64, 64, 64), . . . , (224, 224, 224), (255, 255, 255)). After the measurement, a gamma curve can be plotted (e.g., luminance vs. digital value). FIG. 1 depicts an illustrative plot of luminance versus input digital value for use in gamma correction of an image.
The gamma curve 10 of FIG. 1 is typically implemented to compensate for incorrect input gamma values.
Digital video capture images or still capture images are gamma corrected by assuming certain gamma characteristics for display systems. For example, for high definition (HD) displays, Rec. 709 gamma is applied for gamma correction of an input device or input images. For displaying images that look-like film images on a display, a proprietary gamma (either a gamma power function curve or lookup table (LUT)) is applied for the gamma correction. However, the gamma characteristic of the displays usually does not match the assumed gamma correction applied for the input space. As such, in a typical characterization method, the input gamma is linearized (inverse gamma correction) and the gamma is corrected again using the display gamma.
FIG. 2 depicts an exemplary flow diagram of a conventional method for gamma correction along with corresponding plots of luminance versus input digital value for each of the steps of the flow diagram. The method of FIG. 2 begins in block 20, in which input gamma is determined from a received image. In block 22, the input is linearized by providing a LUT or function that linearizes the input gamma. Subsequently, in block 24, the gamma changes attributable to displaying the received image on the display are determined and corrected for, in block 26, to provide a display gamma curve or function as depicted in FIG. 1. Plots 30, 32, 34 and 36 of FIG. 2 illustratively depict the processing performed by the corresponding process in blocks 20, 22, 24, and 26, respectively. Plots 30, 32, 34 and 36 illustrate a normalized luminance (y-axis) versus an input digital value (x-axis). Consistent with FIG. 2, a characterization of a display is performed to determine a more accurate representation of input images on the display.
While this methodology is accurate and reliable, it has some significant drawbacks. For example, this method: 1) requires costly instrumentation to perform measurements; 2) requires time consuming methods to measure a plurality of patches; and 3) needs to derive the gamma curve from measurement data, among other things.